1. Field of the Invention
The present invention relates to frequency stabilization circuits, frequency stabilization devices, antenna apparatuses and communication terminal equipment, and impedance conversion elements, and, more particularly, to an antenna apparatus installed in communication terminal equipment such as a mobile telephone, a frequency stabilization circuit included in the antenna apparatus, a frequency stabilization device and a communication terminal equipment including the antenna apparatus, and an impedance conversion element.
2. Description of the Related Art
In recent years, as an antenna apparatus installed in a mobile communication terminal, a casing dipole antenna that includes a metal body (for example, the ground plate of a printed circuit board) in a terminal casing as a radiating element has been proposed in Japanese Unexamined Patent Application Publication No. 2004-172919, Japanese Unexamined Patent Application Publication No. 2005-6096, and Japanese Unexamined Patent Application Publication No. 2008-118359. This kind of casing dipole antenna can have performance similar to that of a dipole antenna by performing differential feeding on two casing ground plates (a ground plate in the casing of a body portion and a ground plate in the casing of a cover portion) in a folding-type or slide-type mobile telephone. Since a ground plate in a casing is used as a radiating element, there is no need to provide a dedicated radiating element and the size reduction of a mobile communication terminal can be achieved.
However, in the above-described casing dipole antenna, the impedance of a ground plate used as a radiating element is changed in accordance with the shapes of the ground plate and a casing and the position of an adjacent metal body (a closely disposed electronic component or hinge component). Accordingly, in order to minimize the energy loss of a high-frequency signal, it is necessary to design an impedance matching circuit for each model. In folding-type and slide-type mobile communication terminals, the impedances of a ground plate and an impedance matching circuit are changed in accordance with the positional relationship between the casing of a main portion and the casing of a cover portion (for example, in the case of a folding-type mobile communication terminal, the closed/open state of a cover portion). Accordingly, a control circuit for controlling impedance is sometimes needed.